NATO is ready to defend Turkey in its escalating confrontation with Syria, the alliance's top official vowed Tuesday.

Ankara has sent additional fighter jets to reinforce an air base close to the frontier with Syria, where shells killed five Turkish civilians last week, the Associated Press reported, sparking fears of a wider regional crisis. Syria has defended its shelling of neighboring Turkey as an accidental consequence of its civil war.

Artillery fire was exchanged for the sixth consecutive day Tuesday, while fighting between Syrian government forces and Syrian rebels raged just across the border.

The comments by NATO Secretary-General Anders Fogh Rasmussen were the strongest show of support to Turkey since the cross-border firing broke out last week.

"Obviously, Turkey can rely on NATO solidarity," he said in advance of a meeting of alliance defense ministers in Brussels. "We have all necessary plans in place to protect and defend Turkey if necessary."

When pressed on what kind of trouble would trigger those plans, he said he could not discuss contingency plans.

Rasmussen said it is up to the United Nations Security Council to take whatever action is necessary to force the Syrian government to work toward a peaceful resolution.

“I strongly regret that so far the Security Council has failed in finding an agreement on a legally binding resolution that could send such a strong message to the Syrian leadership,” he said, the Voice of America reported.

Turkish Prime Minister Recep Tayyip Erdogan, addressing parliament members from his party, reiterated that Ankara will continue retaliating to attacks from President Bashar al-Assad's regime.

"Every kind of threat to the Turkish territory and the Turkish people will find us standing against it," Erdogan said. "Soldiers loyal to Assad fired shells at us, we immediately reacted and responded with double force. We shall never stop responding."

At least 25 extra F-16 fighter jets were deployed at the Diyarbakir air base in the southeastern Turkey late Monday, Turkey's Dogan news agency said, quoting unidentified military sources. The military's chief of staff inspected troops along the border with Syria on Tuesday.